1. Field of the Invention
The present invention relates to a semiconductor non-volatile memory, and particularly to an electrically writable and erasable semiconductor non-volatile memory (also called an EEPROM or Electrically Erasable and Programmable Read Only Memory). The present invention is particularly effective for a semiconductor non-volatile memory using a multi-value technique. Besides, the present invention relates to a semiconductor device including the semiconductor non-volatile memory.
2. Description of the Related Art
In the present specification, an electrically writable and erasable-semiconductor non-volatile memory (EEPROM) indicates all semiconductor non-volatile memories in which electrical writing and electrical erasing are literally enabled, and includes, for example, a full function EEPROM and a flash memory in its category. Unless otherwise specified, the non-volatile memory and the semiconductor non-volatile memory are used to mean the EEPROM. Besides, the semiconductor device indicates all devices functioning by using semiconductor characteristics, and includes, for example, a microprocessor, an electro-optic device typified by a liquid crystal display device and an EL display device, and an electronic equipment incorporating a microprocessor or an electro-optic device in its category.
In recent years, an electrically writable and erasable semiconductor non-volatile memory (EEPROM), especially a flash memory has attracted attention as a promising candidate of a memory substituting for a magnetic disk or a DRAM. Above all, a so-called multi-value non-volatile memory each memory element of which stores data of three or higher values has attracted attention as a large capacity memory.
The non-volatile memory is divided into types, such as a NOR type, a NAND type, an AND type or a DINOR type, based on differences in their circuit structures and operation methods. As a memory element constituting the non-volatile memory, there is known a memory transistor including a floating gate, a memory transistor including a cluster layer, a memory transistor having MNOS (Metal-Nitride-Oxide-Semiconductor) structure or MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor) structure, or the like.
In a conventional non-volatile memory, a hot electron injection method (called an HE injection system) and a charge injection method by FN tunnel current (called an FN current system) can be cited as a typical writing operation. In the HE injection system, a high control gate voltage and a high drain voltage are applied to a memory transistor to cause impact ionization, and a generated hot electron is drawn into a gate electrode side, so that an electric charge is injected into the memory transistor. On the other hand, in the FN current system, a high voltage is applied between a control gate electrode and a substrate to cause an FN tunnel current to flow, so that an electric charge is injected into a memory transistor.
In either case, in order to confirm that a threshold voltage after writing is within a predetermined range, verify writing is normally carried out. Especially, in a multi-value non-volatile memory, since it is necessary to control the threshold voltage after writing with high accuracy, the verify writing is indispensable. The verify writing is a method in which a small amount of charge injection and readout for confirming the threshold voltage are alternately carried out. This operation is repeated until the threshold voltage after writing falls within a predetermined range.
In the foregoing method of charge injection, there has been a problem that it is difficult to control the amount of charge injection with accuracy. This is because even in the case where the same operation voltage is applied to the memory transistor, if the charge storage amount of the memory transistor, that is, the threshold voltage is different, the charge injection speed is different.
Thus, naturally in memory transistors having different threshold voltages, even in the same memory transistor, the charge injection speed is changed as time elapses. For example, in the case where an electron is injected, a threshold voltage is increased, and an effective gate voltage seen from a channel region is decreased, so that the charge injection speed is decreased.
Further, if the charge injection speed fluctuates by a factor such as fluctuation in film quality of a tunnel oxide film, even if threshold voltages of memory transistors before writing are equal to each other, a difference gradually occurs in the charge storage amount. Then, a vicious circle occurs in which when a difference occurs in the threshold voltage, a charge injection amount further fluctuates.
The verify writing is one of methods for solving such problems. In the verify writing, since confirmation of the threshold voltage is carried out each time a small amount of charge is injected, it is not necessary to control the charge injection amount with high accuracy. However, in the verify writing, the operation of charge injection is divided into parts, and a readout operation is carried out between them, so that there is a problem that it essentially takes a period of time.
Incidentally, also in the verify writing, it is important to control the charge injection amount with accuracy. By controlling the charge injection amount with high accuracy, the number of times of division of the charge injection operation can be decreased, and so a verify writing time period can be shortened.